In particular, the present invention relates to a liquid management system that enables the pressure of the supplied liquid to be controlled in order to, for example, prime a liquid delivery device, and/or in which the supply of liquid can be provided at a controlled pressure to a liquid ejection location. The liquid may have solid particles suspended or dispersed within it or have other additives added to it, but in all cases the end result is a fluid that behaves substantially like a liquid. Further, the present invention relates to a liquid management system for supplying or receiving liquid comprising heavily sedimenting particles, such as glass frit and/or ink pigment or other solids not well dispersed in the liquid.
In various liquid delivery devices such as inkjet printers or spray heads, it is necessary to achieve a consistent ejection of liquid from the liquid delivery device and/or in order to do so, precise control of the static pressure of liquid is required at the ejection location. Precise control of the liquid flow may also be required. Printheads of the type requiring the properties above are described in EP 1224079 and EP 1366901, for example. Other devices which have similar requirements are disclosed in, for example, EP1071559. EP 2076395 teaches that the pressures at the printhead described in EP 1224079 and EP 1366901 need to be corrected to about + or −20 Pa and those periodic variations must be below about + or −2 Pa to eliminate visible variations in print quality. Likewise, printheads of other designs will be able to tolerate pressure fluctuations (and fluid flow rates) dependant upon their design.
A simple method of controlling the pressure of the liquid supply to a liquid delivery device, such as a printhead, is to use gravity. A liquid reservoir, whereby the surface of the liquid is open to atmospheric pressure, is mounted either above or below the level of the printhead in order to generate a positive or negative liquid pressure, as required by the printhead. The required inlet pressure in the ejection location can be set by mechanically adjusting the relative height of the liquid reservoir with respect to the printhead. The reservoir may also be supplied with liquid by a pump.
Some liquid delivery devices require ink to flow continuously through the device and this requires the device to have both an inlet and outlet to allow ink to flow in and out of the device. In these devices, the pressure of the ink at this outlet can also be controlled by gravity by allowing ink to flow to atmospheric pressure from the outlet tube to a defined level below the printhead. This level can also be mechanically adjusted to achieve the correct operating conditions (such as ink pressure and flow rate) at the ejection location.
As described in EP 2076395, known disadvantages of a gravity-fed ink supply system (which can be generalised to a liquid supply system) are:                Changing the pressures requires physical movement of the reservoirs.        The location of the reservoirs is determined by the required pressures.        A large volume of space may be required to accommodate the total adjustable range of the reservoirs.        Priming printheads with ink can be assisted by supplying ink at pressures that are very different from the pressures required during printing.        With a gravity fed system a large amount of space and typically a significant amount time is required to move the reservoirs to achieve these pressures.        The surface of the ink must be open to the atmosphere, increasing the risk of dust or other contaminates polluting the ink.        
WO 97/441914 and EP 1092548 each describe ink supply systems in which the ink surface is maintained at a constant level or height in the reservoir by use of a weir. Such a system is also described in WO 2006/030235. Such systems can either use gravity to set the pressure of the ejection location or, in the case of WO 2006/030235, the pressure of the ink at the inlet and outlet of a nozzle containing fluid supply apparatus is controlled by controlling the pressure of air above or with air at the inlet and the outlet from the nozzle containing fluid supply apparatus. In order to maintain the functioning weir it is necessary to remove the ink that has flowed over the weir from the reservoir.
EP 2076395 describes a further system in which the ink is maintained at a constant height in the reservoir by use of a weir. In this system, ink is pumped continuously from a remote ink tank to two reservoirs, one placed just before the printhead in the fluidic circuit and one just after. The pressure of the fluid in the reservoirs is controlled such that the ink flows through the printhead at a controllable pressure and flow rate.
In EP 2076395, it is claimed that it is convenient to measure the pressure in the local reservoir by using a gas pressure sensor mounted above the ink level in the reservoir. Therefore, to control the pressure of the ink in the reservoir based on this pressure management, it is important that the depth of the ink in the reservoirs is kept constant.
EP 2076395 uses a weir over which the excess ink pumped into the reservoir flows in order to keep the fluid at a constant height on at the upstream side of the weir. The fluidic path to and from the printhead comes from this ink stored at the upstream side of the weir. The ink that flows over the weir is pumped back to the remote ink tank via a return pump. This return pump is over driven, such that it sucks some air in addition to ink out of the reservoir, thus creating a slight vacuum in the reservoir. The gas pressure sensor and proportional valve are operated in a feedback loop in order to let air leak into the reservoir (usually at atmospheric pressure from outside the reservoir, or alternatively from a positive or negative pressure reservoir) at a rate sufficient to enable the fluidic pressure in the reservoir to stabilise at a user controllable set pressure.
However, when using liquids which comprise heavily sedimenting or poorly dispersed particulates, such a system described in EP 2076395 has a number of drawbacks. These include:                the presence of a weir (especially in the configuration shown in EP 2076395) creates a flow pattern that leads to areas where the flow of liquid is sufficiently low to allow the particles in the liquid to start falling out of suspension. This changes the composition of the liquid such that the liquid delivered to the head is different from that desired. The sediment may also start to fill the reservoir, disrupting or blocking the fluid flow.        A heavily sedimenting liquid typically requires a higher flow rate than non-sedimenting liquid through the printhead and local reservoirs so as to prevent sedimentation when liquid is supplied to a system such as that in EP 2076395, turbulence is created in the reservoir such that the height of the liquid surface above the bottom of the reservoir becomes unstable. Turbulence itself also causes unpredictable variations in fluidic pressure. This in turn causes the pressure of the liquid supplied to head to fluctuate and become difficult to control, even with the feedback system described above.        
The present invention addresses one or more of the problems identified above.